Synthesis and characterization of biopolyurethane crosslinked with castor oil-based hyperbranched polyols as polymeric solid–solid phase change materials

Novel crosslinking bio polyurethane based polymeric solid–solid phase change materials (SSPCM) were synthesized using castor oil (CO) based hyperbranched polyols as crosslinkers. CO-based hyperbranched polyols were synthesized by grafting 1-mercaptoethanol or α–thioglycerol via a thiol-ene click reaction method (coded as COM and COT, respectively). Subsequently, the three SSPCMs were synthesized by a two-step prepolymer method. Polyethylene glycol was used as the phase change material in the SSPCMs, while the CO-based hyperbranched polyols and two types of diisocyanate (hexamethylene diisocyanate (HDI) and 4,4'-diphenylmethane diisocyanate) served as the molecular frameworks. Fourier transform infrared spectroscopy indicated the successful synthesis of the SSPCMs. The solid–solid transition of the prepared SSPCMs was confirmed by X-ray diffraction analysis and polarized optical microscopy. The thermal transition properties of the SSPCMs were analyzed by differential scanning microscopy. The isocyanate and crosslinker types had a significant influence on the phase transition properties. The SSPCM samples prepared using HDI and COT exhibited the highest phase transition enthalpy of 126.5 J/g. The thermal cycling test and thermogravimetric analysis revealed that SSPCMs exhibit outstanding thermal durability. Thus, the novel SSPCMs based on hyperbranched polyols have great potential for application as thermal energy storage materials.

Preparation of CO-based hyperbranched polyols. The synthesis of CO-based hyperbranched polyols was carried out according to the method used in our previous studies 52,53 . The preparation scheme is shown in Scheme 1. CO, thiol, DMPA (as a photoinitiator), and EA (as a solvent) were placed in quartz tubes and rolled in a tube roller for 24 h after placing it in a photochemical reactor equipped with UV-A lamps. The molar ratio of the thiols to C=C double bonds of CO was set to 4:1. After the reaction, the products were washed with distilled water and aqueous NaCl solution at least five times. The obtained polyols were dried using MgSO 4 , and remained organic solvent was eliminated by rotary evaporation. The obtained polyols were then vacuum dried for 24 h. The CO-based polyols grafted with 2-mercaptoethanol and α-thioglycerol were coded as COM and COT, respectively.

Preparation of SSPCMs.
As shown in Scheme 2, the synthesis of SSPCMs using PEG as a PCM was conducted using a two-step prepolymer method. First, a predetermined amount of PEG and the two types of diisocyanate (molar ratio of PEG and diisocyanate = 1:2) were separately dissolved in an appropriate amount of DMF, and the diisocyanate solution was added dropwise to the PEG solution with gentle stirring. A few drops of DBTDL (0.05 wt% based on the total weight of final product) were added as a catalyst. This reaction was carried out at 80 °C under N 2 atmosphere for 3 h to obtain isocyanate-terminated prepolymers. Second, three types of crosslinkers were dissolved in DMF in adequate molar ratios and slowly added into prepared prepolymer solutions. After 18 h, the reaction mixtures were poured into a PTFE-coated mold and thermally cured at 80 °C for 24 h in a convection oven. The products were then kept in vacuo at room temperature for 24 h before analysis. The sample codes are listed in Table 1 according to the types of diisocyanates and crosslinkers used.
Characterization. ATR-FTIR spectrometer (Nicolet iS50, Thermo Fisher Scientific) was used to determine the structure of the SSPCMs over the range of 500-4000 cm −1 . The FTIR spectra were recorded at a resolution

Result and discussion
Preparation of polyols. COM and COT were successfully synthesized under the optimized reaction conditions obtained from our previous studies 52, 53 . The FTIR spectra of the newly prepared polyols are shown in Fig. S1. As stated in the supporting information, the chemical structures of the synthesized COM and COT are consistent with the results of our previous studies 52, 53 Fig. 1. The disappearance of the isocyanate peak at 2265 cm −1 in all the SSPCM spectra indicates that the PUs were successfully synthesized. The newly formed absorbance bands at 3341 and 1539 cm −1 correspond to the N-H stretching and amide vibration within the PU bond, respectively. The FTIR analysis did not show a significant difference in the spectra in terms of the functionality of the crosslinker or the type of isocyanate.
Crystalline structure analysis of the SSPCMs. The crystal structures of the SSPCMs were analyzed by XRD analysis, and the results are presented in Fig. 2. Two strong diffraction peaks were detected in all the SSP-CMs spectra, indicating that the SSPCMs had crystalline structures. PEG is known to exhibit sharp diffraction peaks at 19.1° (120) and 23.3° (032) implying that it is a semi-crystalline polymer with high crystallinity. The XRD patterns of the SSPCMs also showed peaks at 19.1° (120) and 23.3° (032), suggesting that the crystallinity of SSPCM is dominated by crystallinity of PEG. Compared to the XRD pattern of PEG, the SSPCMs had a lower peak intensity and a broader full width at half maximum. The crosslinked structure formed by hyperbranched polyols partially confined the PEG chain, which led to a decrease in the degree of crystallinity. Meanwhile, a significant difference in the degree of crystallinity was observed depending on the isocyanate type used. For PEG, the peak corresponding to the (032) plane in the XRD pattern had a higher intensity than that to the (120) plane. However, for the SSPCMs prepared using MDI, the intensity of the peak corresponding to the (032) plane decreased; for the SSPCMs prepared using HDI, the crystal structure of the pristine PEG was retained as the functionality of the crosslinker increased. The reduced intensity of the peak corresponding to the non-equatorial (032) plane of the SSPCMs prepared using MDI implied that thinner PEG lamellae and tilt of the PEG chain in  www.nature.com/scientificreports/ the lamellae were generated with the formation of the crosslinked structures 54 . This was more prominent for the SSPCMs prepared using MDI because the rigid benzene ring further confines the PEG chain 41 . For further analysis of the crystal structure of the synthesized SSPCMs, the crystal morphologies of the pristine PEG and the SSPCMs were recorded through POM, and the results are shown in Fig. 3. All SSPCM samples demonstrated cross-extinction patterns caused by polarized light. As shown in Fig. 3a, the crystal of pristine PEG grew to several millimeters in size, while the synthesized SSPCMs demonstrated a decrease in the crystal size because the crosslinked structure limited the motion of the soft segment. The crystal structure remained unchanged when the temperature of the SSPCMs was raised to the phase transition temperature. However, as the temperature reached the transition point, the crystal structure began to disappear and eventually no crystal features could be observed in the POM image ( Fig. 3e and i). As shown in Fig. 4, the SSPCMs remained in the solid state even at a high temperature close to the phase transition temperature. The solid-liquid phase transition did not occur in all the synthesized SSPCMs, even at temperature higher than the transition point.
Phase transition properties of the SSPCMs. DSC analysis was performed to examine the phase transition properties of the synthesized SSPCMs. The five consecutive DSC scans after heating and cooling of the pristine PEG and SSPCMs are presented in Figs. S2 and 5, respectively. For the pristine PEG, the solid-liquid phase transition resulted in very sharp exothermic and endothermic peaks in the range of 20-70 °C. The DSC scans for all SSPCMs also exhibited distinct exothermic and endothermic peaks, suggesting that the SSPCMs had reversible thermal storage and release properties. A consecutive heating-cooling test was performed to verify the potential reusability of the SSPCMs. The DSC thermograms remained unchanged during the test. The phase transition properties based on the five consecutive DSC scans are summarized in Table 2. The exothermic and endothermic enthalpies of all SSPCM structures were significantly lower than those of the pristine PEG. This implies that the heat storage and release capacities of the synthesized SSPCMs were lower than those of the pristine PEG. The solid-solid phase transition of the SSPCMs is due to the transformation of the PEG soft segment from a crystalline to an amorphous state. The PEG chains in the SSPCM structures were strongly confined to a finite interspace because of the crosslinked network formed by the hyperbranched crosslinker. Thus, the crystallization of the PEG chain was limited and certain PEG chains were prevented from crystallization in the phase transition process. Therefore, the crystal domain of the SSPCMs was reduced compared to that of the pristine Regarding the influence of the functionality of the crosslinker on the changes in the SSPCMs endothermic enthalpy, the degree of latent heat for both MDI-and HDI-based SSPCM followed the order; CO < COM < COT. Studies have revealed several factors that influence the SSPCM phase transition properties, including soft segment content by weight, crystalline state of the soft segments, and steric hindrance of the crosslinking points. With an increase in the functionality of the crosslinker, the synthesized SSPCM can accommodate more PEG, thereby increasing the latent heat storage capacity. For better understanding of the phase transition properties of the SSPCMs on the basis of the functionality of the crosslinker, the relative latent heat efficiency was calculated using the following equation: where H m.SSPCM and H m.PEG indicate the melting enthalpies of SSPCMs and the pristine PEG, respectively, and ω PEG is the mass fraction of PEG in the SSPCM. Table 2 presents the relative latent heat efficiency of the SSPCM. The η value indicates the influence of the framework structure, where higher values suggest less heat loss of the PEG in the SSPCM. Both MDI-and HDI-based SSPCMs exhibited η values in the order of CO > COM > COT, suggesting that the increased level of steric hindrance caused by the increased crosslinking density led to an increase in the number of soft segments that cannot form the crystal structure during the phase transition process.
Considering the influence of the isocyanate type on the endothermic enthalpy of the SSPCMs, the latent heat of the HDI-based SSPCM was higher than that of the MDI-based SSPCM when an identical crosslinker was used. The η value of H.CO, H.COM, and H.COT were shown to be increased by 3.0%, 3.8%, and 3.5%, respectively, compared to M.CO, M.COM, and M.COT, which is due to the presence of a rigid benzene ring in MDI that prevents PEG crystallization.
The analysis of the phase transition properties of the SSPCMs by DSC demonstrated that using hyperbranched polyols led to an increase in latent heat efficiency, although the relative latent heat efficiency decreased due to an increase in the crosslinking density. Steric hindrance arising from the structural characteristics of isocyanate is another factor influencing the latent heat efficiency of the SSPCMs.
The results of the crystalline structure and phase transition property analyses confirmed that the synthesized SSPCMs exhibited repetitive heat storage and release characteristics without any leakage. The phase change mechanisms of the SSPCMs crosslinked with CO and COT are shown in Fig. 6. The crosslinked networks formed by the CO-based hyperbranched polyols served as the molecular framework, which prevented the dissolution and flow of the PEG soft segment. Compared with the CO-based SSPCM, the COT-based SSPCM exhibited higher thermal energy storage and release efficiencies because a greater amount of the PCM material can be integrated into it. Additionally, Table 3    cycling test (100 cycles from 0 to 100 °C) was performed. Even after the 100th thermal cycle, no weight reduction was observed in the SSPCMs, which indicates that they exhibit thermal reliability even after repeated use within the phase transition temperature range. This result verified the applicability of the SSPCMs as PCM materials. Figure 7 shows the FTIR results for the SSPCMs after 100 cycles of the thermal cycling test. The initial FTIR curves for the SSPCMs are represented by dotted lines. Even after 100 cycles of the thermal cycling test, the FTIR     www.nature.com/scientificreports/ curves for the SSPCMs displayed almost identical shapes and locations of the absorption peaks, suggesting that neither thermal decomposition nor structural changes occurred during repetitive thermal cycling. Figure 8 shows the five consecutive DSC heating-cooling scans for the SSPCMs after the 100-cycle thermal cycling test; the phase transition characteristics are listed in Table 4. All the SSPCMs demonstrated distinct exothermic and endothermic peaks even after thermal cycling test and exhibited with a relatively uniform thermal behavior. These results confirmed that the SSPCM structures retained their reversible heat storage and release properties. However, the melting enthalpies decreased for all samples, and the degree of reduction of relative latent heat efficiency varied based on the isocyanate and crosslinker types. Figure 9 shows the initial relative latent heat efficiency of the SSPCMs followed by the final relative latent heat efficiency and the degree of reduction in the relative latent heat efficiency after thermal cycling. MDI-based SSPCMs exhibited a greater decrease in efficiency than HDI-based SSPCMs. Regardless of the isocyanate type, the reduction of efficiency gradually became smaller as the crosslinker functionality increased; M.COT and H.COT demonstrated reductions by 4.1% and 3.3%, respectively. Thus, hyperbranched polyols are feasible for use as crosslinkers for the production of highly efficient and durable SSPCMs.

SSPCMs Heating rate (°C/min) T m (°C) ΔH m (J/g) T f (°C) ΔH f (J/g) References
TGA was performed to evaluate the thermal stability of the SSPCMs; the results are presented in Fig. 10. The weight of the pristine PEG and SSPCMs remained unchanged up to approximately 280 °C, implying that the SSPCMs were highly stable in the phase transition temperature range, facilitating TES application. Moreover, all the SSPCMs exhibited thermal decomposition at temperatures higher than those for the pristine PEG, which can be attributed to the crosslinked molecular structure of the SSPCMs. The derivative TGA curves in Fig. 10b shows that the SSPCMs exhibited two phases degradation behavior in contrast to the phase 1 thermal degradation behavior of the pristine PEG. Phase 1 is marked by the thermal degradation of the urethane bonds in the SSPCMs, whereas phase 2 is represented by the degradation of PEG 37,65 . Also, the maximum decomposition temperature of all the SSPCMs was higher than that of PEG, exhibiting the outstanding thermal stability of the SSPCMs.

Conclusion
In this study, two types of hyperbranched polyols, COM and COT, with an increased hydroxyl value, were prepared via the thiol-ene click reaction with CO. A new SSPCM series for TES was successfully prepared using PEG as the PCM in the SSPCMs, while CO, COM, and COT provided the molecular framework. FTIR analysis of the SSPCMs revealed the formation of PU structures, which confirmed the successful synthesis of the SSP-CMs. Moreover, the results of the XRD analysis and POM indicated the solid-solid phase transition. The results of the DSC analysis revealed that the isocyanate and crosslinker types had a significant influence on the phase transition properties. The H.COT exhibited the highest phase transition enthalpy at 126.5 J/g. Furthermore, the results of the thermal cycling test and TGA demonstrated the outstanding durability of the SSPCMs. Thus, the novel SSPCMs based on hyperbranched polyols has great potential to be applied in TES materials.